There are two ways you can move Project(s) from one Portfolio to another.
Portfolio Page
The first way allows you to move multiple Projects in the same Portfolio at the same time.
Locate the Project(s) in the Portfolio space.
Click the checkbox on the Project(s) that you want to move. Click Move in the menu
Select the Portfolio you want to move the Project(s) to.
Project Info Modal
The second way to do this is in the Project Info Modal.
If you only need to move one project, you can get here by clicking on the info icon on the Project from various places:
Portfolio Page
Project Page
Project Drawer
Here, you can select the Portfolio you want the project moved to.